Sensors are commonly used to measure current or voltage at a particular point in a system and to provide output signals indicative of the sensed current or voltage. In one typical application, a current sensor is used as part of a feedback control circuit for accurately controlling motors in precision equipment, such as automated assembly robots or numerical control machines. The current sensor detects the current flowing through the windings of the motor and delivers a proportional signal to the control circuit. The torque output of the motor is directly related to the current flowing through its windings. Thus, based on the signal from the sensor, the input to the motor can be altered or maintained by the control circuit to precisely control the torque of the motor.
In such applications, the control circuit requires extremely accurate sensing to precisely vary the input to the motor. The output signal from the sensor should ideally be linearly related to the sensed signal and have a well-specified gain to drive downstream circuitry in the feedback loop. Due to large voltages that are present on the motor windings, the sensor output should also ideally be completely isolated from the input to protect from voltage transients or surge currents which could detrimentally affect the control system. Moreover, the sensor should require limited power consumption, be small in volume, and have minimal requirements for cooling. Each of these features must be consistent over varying temperature. Since space is often a constraint, the sensor should also occupy as little space and volume as possible.
In light of these constraints, various sensors have been developed for accurately sensing motor current. Each has advantages and disadvantages. One widely-used type of sensor for precision motors is the Hall effect sensor. This device utilizes the Hall effect to generate a voltage linearly related to the changing current in the motor. The device is comprised of a semiconductor wafer having a DC power source connected to opposite ends of the wafer. The wafer produces voltage proportional to the intensity of the applied magnetic field, which itself is proportional to the motor current. Although this type of sensor provides accurate readings without large power consumption on the input side, power dissipation on the output side can be very large. Moreover, the output of the sensor varies drastically with changing temperature due to the inherent properties of the semiconductor material. Furthermore, due to the relatively large size of the magnetic core, the Hall effect sensor cannot be easily implemented in a small volume.
To lessen the amount of volume required for sensing, certain isolation amplifiers have been developed which do not rely on the Hall effect for sensing and thus do not require a magnetic core. These amplifiers sense a voltage developed across a current-sensing resistor at the input section of the amplifier and transmit the signal non-galvanically to a receiver section of the amplifier, thereby minimizing the interaction of the circuit that follows the amplifier with the circuit that precedes it. These circuits have the advantage of accurate signal sensing capability at constant temperature. However, some of these amplifiers deliver an output and have a gain which varies due to temperature and process variations. Moreover, these circuits, being largely electrical, may require a supply of power which impinges on system requirements, especially if bipolar integrated circuitry is used. Larger power supplies can be constructed to produce sufficient power, but such supplies would add cost and volume to the system. As an alternate, CMOS configurations may be used which require less power, but these circuits produce undesirable substrate currents if the input signal swings below ground. DC biasing or rectification techniques may be used to boost the signal above ground, but such techniques may add errors to the amplifier output which detrimentally affect signal-sensing accuracy.
In addition to preventing the disadvantages discussed, an ideal amplifier should also provide an isolation barrier sufficient to prevent ripple in the output of the amplifier caused by voltage transients and surge currents. In more extreme cases, if the isolation barrier is insufficient, such transients or surges may damage the output circuit of the amplifier or the circuits that follow.
Several solutions have been proposed to establish a sufficient isolation barrier within an amplifier which does not rely on magnetic effects, but each has caused disadvantages in overall sensor performance. In one proposed solution, disclosed in U.S. Pat. No. 4,843,339 issued to Burt et al , capacitors are used to realize the isolation section of the amplifier. Such capacitor isolation, however, suffers from the inability of a capacitor to block rapidly changing voltage inputs (high dV/dt), such as high-frequency voltage transients. This inability limits amplifiers with capacitor isolation from being used for noisy applications.
Optocouplers, also known as optoisolators or optical isolators, have also been proposed for isolation. Optocouplers convert an electrical input signal to an optical signal, transmit the optical signal, and restore the transmitted signal to an electrical output signal similar to the electrical input signal. Although optical isolation is strong and provides superior transient rejection as compared to capacitor isolation, the LED used to realize the transmitter section of the optocoupler causes disadvantages. Due to nonlinearities of the LED, errors are introduced into the analog signals at the detector end of the coupler. If digital transmission is used, errors due to nonlinearities can be reduced, but the LED will have to be turned on and off, resulting in large power supply fluctuation which can cause undesirable system effects.
It is, therefore, desirable to provide an isolation amplifier for accurately sensing voltage or current, which provides a linear output, has complete isolation, and a well-specified gain, each of which does not vary with changing temperature or fabrication processing.
It is also desirable to construct the amplifier out of CMOS devices without having to rectify or bias the input signal.
It is further desirable to provide such an amplifier with strong optical isolation which does not require a large amount of power or power supply fluctuation.
It is still further desirable that the isolation amplifier be relatively inexpensive and that the complete isolation amplifier fit into a conventional 8-pin dual in-line package (DIP) which occupies a minimum amount of space and volume.